1. Field of the Invention
This invention relates to an image forming apparatus for a printer or a copying machine which forms an image using techniques such as an electronic photographic technique or an inkjet technique, and a control method of the image forming apparatus.
2. Description of the Related Art
In recent years, there is increasing demand for direct imaging printer which does not need a printing plate used in off-set printing or the like. Many companies use direct imaging printers in order to reduce time required for printing, realize services adapted for individual customers, satisfy the demand for mass-printing, and address environmental problems involving discard of sheets with printing errors. Among the direct imaging printers, the ink jet printer suitable for photographic printing, which is advantageous in prices, and the electronic photographic printer, which is high in productivity and near to the offset printing in quality, are increasing in market share.
Under these circumstances, the most important function among those functions required for a direct imaging printer as an alternative to conventional offset printing or photograph is to maintain stability of colors of an image formed on a sheet.
In order to ensure stability of colors, various manufacturers have proposed techniques which enable a direct imaging printer to carry out color stabilizing control (without intervention by a control of an external device such as a computer). More specifically, there is disclosed a technique in which a pattern of a toner patch image for use in detecting toner density is formed on a surface of a photosensitive member in an electronic photographic printer and is read by a density sensor, and the resultant reading information is fed back from the density sensor to a toner density controller of a developing unit that carries out control so as to produce the appropriate toner density (for example, see Japanese Laid-Open Patent Publication (Kokai) No. H01-309082).
Although the toner patch image is generally easily formed and cleared, only toner density information before the toner image is fixed on a sheet can be obtained. Therefore, when the toner density control is executed based on the toner density information, influences after the fixing process cannot be reflected on the toner density control.
Thus, in a copying machine, for example, there has been proposed a method for causing a reader unit provided in the body of a copying machine (printer unit) to read an image formed on an output sheet by a printer unit, and for performing an image control based on the result of image reading (for example, see Japanese Laid-Open Patent Publications (Kokai) Nos. S62-296669 and S63-185279). With this method, however, a user is required to perform complicated operation such as picking up an output sheet, on which an image is formed by the printer unit, from a sheet discharge section, feeding the output sheet in the reader unit, and setting the reader unit to be ready for image reading. Because of the complexity of operation, some users omit the operation which should be done periodically.
In order to eliminate the complexity of operation, there has been disclosed a technique of setting a sensor in the midway of a conveying path extending downstream of a fixing device for fixing a toner image on a sheet, and detecting an output image formed on the sheet (toner patch image) (for example, see Japanese Laid-Open Patent Publications (Kokai) Nos. H10-193689, H11-231585, and 2000-241242). Further, there has been disclosed another technique in which achromatic color balance (gray balance) to which human eyes are sensitive is adjusted based on output image's color (R, G, B) being detected (for example, see Japanese Laid-Open Patent Publication (Kokai) No. 2002-344759).
On the other hand, the ink jet printer has a problem that colors of inks printed on a sheet vary, though not so much as in the electronic photographic printer, due to variation in the amount of discharged ink with passage of time, differences in environment, or individual differences between ink cartridges. Therefore, also in the field of ink jet printers, a printer has been put on the market, which has a density sensor disposed next to an ink head so that the color stability after the ink is printed on a sheet may be recognized and controlled with accuracy.
As described above, in the direct imaging printer, the most important problem is to maintain color stability irrespective of whether the printer is based on the electronic photographic technique or the ink jet technique. Printer manufacturers must guarantee the color stability for users. It is important for printer manufactures to commercialize products produced not only in consideration of technical improvements but also in consideration of users' operability. Attention is much focused on the color stabilizing control of an output image with use of a sensor disposed on a conveying path extending downstream of the fixing device.
However, the arrangement for executing the color stabilizing control of an output image using a sensor set, as in the above-mentioned prior art example, on a conveying path on the side downstream of the fixed device entails the following problems.
If, in an electronic photographic printer, a toner patch image is detected before the image has been fixed on a sheet, reading timing of toner patch image can be determined based on writing timing in which an electrostatic latent image is formed by laser to the photosensitive member. If, on the other hand, a toner patch image is detected after it has been fixed on a sheet by, for example, detecting the toner patch image when a predetermined time has elapsed from the completion of the toner patch image being fixed, erroneous detection may sometimes be caused in a sensor disposed downstream of the fixing device depending on timing of the sheet to enter a sensing area of the sensor, expansion and/or skewing of a sheet.
In order to prevent the above-mentioned erroneous detection, the inventors of the present invention study a method for generating a read timing of a toner patch image by sensors, as described below.
First, an explanation will be given of a case where a maximum density adjustment pattern (toner patch image) is read by a color sensor without using a trigger bar (a band arranged on the left side of the toner patch image), as shown by way of example in FIG. 11A.
In order to generate read timing of a color sensor without using a trigger bar, read timing may be generated with use of a flag (contact) type sensor or an optical sensor used for sheet jam detection. This detection method is, however, affected by a variation in the location of a toner patch image originally formed on a sheet. Considering a cost, although a trigger may be generated on the basis of the result of detection by the color sensor, the above-mentioned problem cannot be solved in this case.
Next, a case where read timing in which a toner patch image formed on a sheet (medium) is read by a color sensor is generated using the density or color contrast of the toner patch image, as a trigger will be described.
An image formation at one end of a maximum density adjustment pattern as exemplarily shown in FIG. 11A should be carried out in a condition using much color material (toner) for strengthening a color contrast compared to that at another end of the pattern. In order to absorb a variation in sheet conveying speed, the toner patch image needs to be bigger in size. This results in a low flexibility in placing the toner patch image on a sheet, and a variation in the color contrast of toner patch image serving as a trigger for the reading action of the color sensor, making it difficult to adapt to detection of a gradation pattern including many low-density pattern portions or to control to determine the amount of color material to be put on the toner patch image by the printer engine.
Next, a method will be described in which the read timing of a color sensor can be generated in the most well-balanced manner using a trigger bar.
This method is effective to relive a weak point in the mechanism of an image forming device such as insufficient sheet registration accuracy (sheet transfer position accuracy) or the presence of variation in sheet conveying speed. In the method, however, if a contrast ratio between a trigger bar and a sheet is less than a certain value, accurate read timing of a color sensor cannot be attained. Since such trigger bar is formed, the number of toner patch images which can be formed on a sheet decreases. A bordering part of toner patch image cannot be detected with accuracy. In particular, in the case of performing control to determine how much amount of color material is to be put on each toner patch image, image formation is carried out while changing an image forming condition for attaining the maximum toner density, and therefore, the trigger bar cannot be formed stably.
A role of a color sensor mounted on the image forming apparatus is to match the maximum toner density and a toner gradation, as described in the above-mentioned Japanese Laid-Open (Kokai) Patent Publications Nos. H01-309082, S62-296669, and S63-185279. When the maximum density is matched to the image forming condition (such as a charged potential of photosensitive member), it is enough to change only development contrast (difference between a potential for forming electrostatic latent image on a photosensitive member and a bias potential). Generally, the development contrast can be changed only by changing the charged potential of photosensitive member with keeping the amount of light or by changing the amount of light with keeping the charged potential of photosensitive member. Changing the amount of light, the latter method, is faster in response and more suitable for color stabilizing control. Thus, many companies are adopting the latter method. In considering color stabilizing control, however, the above-mentioned concept for trigger generation is difficult to be realized.
Namely, if a toner patch image is formed in stages with changing the amount of light in a detected part of the toner patch image, the trigger bar must be formed in the toner patch image with a low amount of light. As a result, as shown in FIG. 11B, a problem occurs in that a color sensor cannot detect any toner patch image in suitable timing.